The border area has already been heavily evacuated, with many sent to Jordan's Zaatari camp, the world's second largest refugee camp with about 112,000 residents, he said.

More than 2 million Syrians have fled the country's two-and-a-half year conflict. Jordan hosts more than 554,000 refugees.

Kessler said Tuesday that Syria's neighbors are also "monitoring" their borders and regulating arrivals. Host nations complain they are bearing a heavy burden as international funding is not forthcoming.